Sicp is not computation first. Sicp is understanding first.
Doing the calculations automatically is a happy side effect of finding the right abstractions for describing what's happening physically and those abstractions being expressed in scheme already.
E.g. Exercise 3.73 in SCIP asks how to implement an electrical circuit using a stream data structure. Because of all the work done beforehand you end up with an expression which describes the time behaviour of the circuit using the same expressions that describe its layout.
Doing the calculations automatically is a happy side effect of finding the right abstractions for describing what's happening physically and those abstractions being expressed in scheme already.
E.g. Exercise 3.73 in SCIP asks how to implement an electrical circuit using a stream data structure. Because of all the work done beforehand you end up with an expression which describes the time behaviour of the circuit using the same expressions that describe its layout.